1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to selecting a track density for each disk surface of a disk drive based on a head characteristic.
2. Description of the Prior Art
FIGS. 1A and 1B show a prior art head disk assembly (HDA) comprising a disk 2 having a top surface 4A and a bottom surface 4B. The disk 2 is rotated about a center axis while respective top and bottom heads 6A and 6B write and read data to/from the respective disk surfaces 4A and 4B. The heads 6A and 6B are attached to the distal end of respective actuator arms 8A and 8B which are rotated about a pivot by a voice coil motor (VCM) 10 in order to actuate the heads 6A and 6B radially over the disk surfaces 4A and 4B. Each disk surface 4A and 4B comprises a number of radially spaced, concentric tracks (e.g., tracks 12A on disk surface 4A) as defined by a number of embedded servo sectors (e.g., servo sectors 14A on disk surface 4A). The embedded servo sectors 14A are read by head 6A and processed by a servo control system in order to maintain the head 6A over a center of a target track while writing or reading data. The embedded servo sectors, 14A may be written to the disk surface 4A using an external servo writer or using internal circuitry during a self-servo writing process.
The overall recording density of each disk surface is determined in part by the track density or radial tracks per inch (TPI). Since the embedded servo sectors define the tracks, the track density is established when the embedded servo sectors are written to the disk during the servo writing process. The track density and therefore the recording density of the disk drive is limited by the characteristics of the head such as the width of the head. In the past, a nominal track density has been selected to servo write all of the disk surfaces corresponding to the average characteristics for the type of head used to manufacture a family of disk drives. If during a “burn-in” process of each disk drive the track density is found to be too high for any one of the heads, the entire disk drive may be discarded as defective, the head/disk replaced, or the respective disk surface marked as unusable.
U.S. Pat. No. 6,091,559 discloses a method for optimizing the track density for each disk surface by measuring a track width written by each head. The track width is measured by writing a first pattern to the disk, and then reading the first pattern to generate a first read signal amplitude measurement A0. A second pattern is then written on both sides of the first pattern at a predetermined radial offset, and the first pattern is read again to generate a second read signal amplitude measurement A1. A track density is selected for the disk surface in response to a ratio between the first and second read signal amplitude measurements A0 and A1. A problem with this technique is that the first pattern is read over two revolutions of the disk: once before the second pattern is written and again after the second pattern is written.
There is, therefore, a need for an improved technique for estimating the optimal track density of each disk surface in a disk drive.